Fill the bath or the sink nearly full with water. Fill the 2 litre container with
water. Put your hand over the opening of the bottle and turn it upside-down under the
water of the bath. Ask a friend to hold the bottle under the water while you slide the
plastic tubing into the bottle. Take a deep breath. Put the other end of the tubing into
your mouth and blow as much as you can. Mark where the air level is in the upside-down
bottle with the waterproof pen. Put the bottle out and empty it. Now measure your lung
capacity by filling the bottle up to the air level mark, and pouring this water into a
measuring jug.

In fact, this is only the Expired Air Capacity. There is always a residual amount of
air in the lungs as well to stop them from collapsing.